Optical systems designed for switching, modulating and amplifying laser light beams often make use of electro-optical materials. In lasers or other optical systems, electro-optical materials are often configured as Pockels cells based on Pockels electro-optic effect where the indices of refraction both of ordinary and extraordinary rays are changed under the applied electrical field. The Pockels effect could occur in materials without the center of symmetry like Rubidium Titanyl Phosphate (RTP), Rubidium Titanyl Arsenate (RTA), Potassium Titanyl Phosphate (KTP), Lithium Niobate (LN), Lithium Tantalate (LT) and other materials.
Pockels cells are implemented in many scientific and technical applications. Pockels cell combined with polarizers are used in many applications and are particularly used to control light switching and/or light modulation. In such applications, the polarization orientation of electro-optical crystal is varied according to a variation of an electric field applied to the Pockels cells, consequently varying the amount of light that passes through the polarizer and creating a shutter/modulator. By either applying or turning off the external electrical field to electro-optic material could switch between 0° optical rotation and 90° optical rotation this shutter is able “to open” and “to close” the light very fast with the rate of a few nanoseconds. The same method is used for beam modulation by the polarization rotation between 0° and 90°, in this case the beam as viewed through the polarizer shows an amplitude-modulated signal.
In other applications Pockels cells are used for laser amplification, a laser medium in a regenerative amplifier cavity is pumped to generate an excess of excited atoms in the medium. Pockels cells are then used for amplification of gain medium by preventing the oscillating light coming out of the cavity. When the Pockels cell is switched the intra-cavity light is able to emit out of the cavity and by such a way creating a fast high energy pulse. This setup could be used for many applications like Q-switching, chirped pulse amplification and cavity dumping. In addition, Pockels cells could be used for quantum key distribution and in electro-optic probes.
The thermo compensated Pockels cell without any additional thermo stabilizing device is based on using two identical crystals. It is known that electro-optical properties of the non linear optical materials are temperature dependent and in order to achieve a stable and reliable operation of the Pockels cell the temperature of the used nonlinear material must be stabilized. Using two crystals allows the stable performance of the Pockels cell without any thermo stabilizing unit. The light propagates is either along the X or Y axes (depending on the crystal cut plane), both of which exhibit birefringence. The two crystals are located in series by such a way that the second (or the first) is oriented at 90° regarding the first (or the second) crystal. In this arrangement any change in the birefringence of the first crystal caused by the temperature is cancelled or compensated by the other crystal keeping the whole Pockels cells without the temperature influence. In order to achieve the perfect matching both crystals should have the same dimensions. In addition, material properties should be similar for both crystals in order to obtain a high contrast ratio of Pockels cell. Moreover, the matching between crystal properties and the mechanical mount are required in order to provide a stable reliable operation of the Pockels cell over the wide temperature range from −60° C. till +120 ° C. The two crystals are mounted on the same electrically conducting base and the upper electrodes are wired together as well so both crystals have the same voltage all the time. Generally, both crystals are glued to the holder by such a way to get the best matching between them and achieve the highest contrast ratio. Once the crystals mounted, there is not any possibility for further optimization that might be required during the operation in the customer end system.
The Pockels cells based on thermo-compensated design are used in many applications where pulses are varied from picoseconds to milliseconds range. It was observed that during operation of Pockels cells with femtosecond range pulses the sideband pulses of the same femtosecond range duration near the main pulse were observed. Generally, for longer pulse these femtosecond sideband pulses might not be observable and thus it has a negligible influence on the main pulse while for femtosecond main pulse the sideband pulses reduce the main pulse intensity. It was observed that slight variations in the offset angle between the two crystals (i.e. slightly above or below 90°) may reduce or even eliminate the sideband pulses. The regular common technique of the two crystals mounting by gluing does not provide the needed optimization for femtosecond range pulses and more precise adjustment is required.
US Patent Publication No. 2007/0236771 discloses a method and system for laser amplification using a dual-crystal Pockels cell. In this system, the Pockels cell is constructed to enable adjustment of the rotational orientation of one crystal relative to the other crystal. The rotational orientation of one or both crystals in the Pockels cell is adjusted to control sidebands in the laser pulse.